Magnetic resonance imaging (MRI) devices apply a main magnetic field through an examination region. This strong field, typically denoted B0, acts to align the nuclei within a subject to be examined. In some MRI devices, the B0 field is horizontally oriented, and in others it is vertically oriented.
In both horizontally and vertically oriented systems, magnetic resonance is excited in the aligned nuclei by a relatively strong orthogonal RF (radio frequency) field, typically denoted B1. The B1 field causes the aligned nuclei or spins to tip into a plane orthogonal to the static magnetic field B0. Over time, the spins realign themselves with the B0 field emitting relatively weak RF resonance signals as they precess. This resonance is detected by RF coils tuned to the specific resonance frequency desired. These resonance signals are passed to image processing equipment to reconstruct the signals into an image representation for display on a video monitor.
Typically, the transmit RF signals are orders of magnitude larger than the magnetic resonance signals generated by the excited nuclei and detected by the RF receive coils. To maintain patient safety and to protect the sensitive receiver equipment including the coils, the receive coils are typically decoupled or detuned during the transmit phase of an imaging procedure. Accordingly, it is known to decouple receive coils using semiconductor switches or PIN diodes in conjunction with LC circuitry using one of two principal variants, namely active decoupling and passive decoupling.
With active decoupling, during the transmit phase of an imaging operation a bias is applied to a PIN diode semiconductor switch in conjunction with an LC circuit to decouple or detune the coil. As technology has improved and the power of the transmit RF pulses has increased, increasingly higher bias currents on the switching diodes have been used to ensure the receiver coil remains decoupled. Unfortunately, these higher bias currents—in addition to increasing design complexity and heat dissipation in DC supply lines—introduce magnetic field distortions in the B0 field close to the subject degrading the image obtained.
With passive decoupling, antiparallel diode semiconductor switches in conjunction with LC circuitry are also employed. In this method, antiparallel combinations of high speed switching diodes decouple the coil in response to the transmit pulse itself. In other words, when the antiparallel combination of diodes is exposed to the high power transmit signal, each diode conducts during its respective half cycle. This allows high currents, but not low currents, to see a parallel resonant LC circuit which decouples the coil. While this method employs no bias currents and eliminates the associated B0 field distortions, the coil is always decoupled during the RF transmit pulse and always coupled or active during receive.
A receiver coil in a magnetic resonance imaging system is a sensitive antenna, whose function is to receive the electromagnetic signal originating in the patient's tissue during an imaging sequence. However, in order to get this echo signal, the system first emits a strong electromagnetic pulse, that carries energy to the protons in the tissue. This pulse also couples to the receiver coil, which has negative impacts: 1) the pulse is affected, which degrades the image quality, 2) induced voltages in the receive coil may generate electromagnetic fields risking patient safety, and 3) induced voltages may break the receiver coil. It is required that the patient safety must not be compromised due to the presence of the coil, independent of it being connected to the system or not.
To minimize the abovementioned coupling of the transmit pulse, the receiver coil contains specific detuning circuits, whose function is to decouple the receiver antenna loops from the emitted transmit pulse. The detuning circuitry must be such that is prevents the transmit pulse from coupling, but doesn't degrade the receiving performance of the coil too much.
In order for the coil to remain safe even when it is left unconnected, at least a part of the detuning circuitry needs to be passive, i.e. it must keep the receiver coil safe to the patient without system control. However, during normal operation, passive detuning circuits can be suboptimal in image quality point of view, and thus also active detuning is usually implemented in the coil, or the passive detuning circuitry can also be actively controlled by the system when the coil is connected.
Several methods of realizing a detuning circuit are presented in U.S. Pat. No. 6,850,067. A detuning circuit is a parallel resonance circuit that is connected in series with an MRI (Magnetic Resonance Imaging) receiver coil antenna loop. The circuit creates high impedance to the current flowing in the antenna loop, thus decoupling the antenna from the surrounding electromagnetic field. While this is favorable only during the transmit pulse, the parallel resonance circuit must be such that it can be disabled (made low-impedance) during the receiving period. This can be done with a PIN diode connected in series with one of the resonance circuit components, such that only when the PIN diode is conducting, the resonance circuit is in the high-impedance state. The PIN diode can be driven conductive either with an auxiliary DC current supply (active detuning circuit) or an internal circuit that utilizes the transmit field energy to forward bias the PIN diode (passive detuning circuit).
In a passive detuning circuit, the biasing current of the PIN diode is generated by taking some RF power from the current flowing through the detuning circuit and converting it to a DC current flowing through the PIN diode. Depending on the design of the circuitry that does this conversion, the biasing current might be too low or unstable to maintain the PIN diode series resistance low enough. If the forward bias is not high enough or is intermittent, the PIN diode starts to dissipate more heat and also degrade the overall performance of the detuning circuit. Due to this, the conversion circuit must be as efficient as possible.
The simplest solution to feeding a DC current through the PIN diode is to connect a high-speed rectifier diode antiparallel to the PIN diode, so that when the PIN diode is not conducting, the negative phase of the current flows via the rectifying diode, thus creating an average positive current flowing through the PIN diode. The paradox is that if the PIN diode was driven perfectly conductive, no RF voltage would be seen across the antiparallel rectifier diode, and thus no current would flow via this rectifying diode, and no forward-biasing DC current would flow through the PIN-diode. As a conclusion, the PIN diode would continuously be balancing between the non-conducting and conducting state, thus dissipating more than necessary.
Some of the solutions explained in U.S. Pat. No. 6,850,067 use a way of getting the RF current not from across the PIN diode, but across the whole detuning circuit. In this way, the better the detuning circuit works (PIN diode conducts well), the higher the voltage available across the detuning circuit terminals and thus the rectifying circuitry. The current is drawn through a current-limiting capacitor to a rectifier pair, which creates the DC current for the PIN diode. The idea is good in theory, but realistically not implementable. The two rectifying diodes in series are connected antiparallel to the PIN diode, effectively exposing them to the same conditions as the single rectifier diode used in the simplest solution explained above in the beginning. Thus no existing fast rectifier can be used due to their low power and reverse voltage handling capabilities.